This application relates to the electrochemical detection of analytes in a test sample. The invention is particularly applicable to the detection of glucose in a sample of blood or other biological fluid using a disposable electrochemical test strip.
FIG. 1 shows a schematic representation of a conventional enzyme biosensor for the electrochemical detection of glucose. The biosensor has a working electrode 10 and counter electrode 11 in a circuit with a voltage source 12. The sample is disposed between the electrodes 10,11 to complete the circuit. When glucose is present, glucose oxidase, GOXox present in the sensor oxidizes the glucose to gluconolactone and reduced enzyme, GOXred is formed. An oxidized mediator, MEDox, present in the sensor restores the enzyme to the active oxidized form, GOXox, and generates reduced mediator, MEDred. The applied voltage in the circuit is selected such that reduced mediator is oxidized at electrode 11 and that oxidized mediator, MEDox, is reduced at electrode 10. Current flow within the sensor results from the oxidation and reduction of the mediator at the electrodes, and this current flow is frequently measured to assess the amount of glucose in the sample. The biosensor shown in FIG. 1 can be modified for use with other analytes, for example by selection of a redox enzyme (oxidase, dehydrogenase etc.) with different specificity appropriate for the other analyte.
Numerous mediators have been disclosed for use in biosensors of the type shown in FIG. 1. In general, suitable mediators are ferricyanide, metallocene compounds such as ferrocene, quinones, phenazinium salts, redox indicator DCPIP, and bipyridyl-substituted osmium compounds. See, for example, U.S. Pat. Nos. 5,589,32, 6,338,790 which are incorporated herein by reference. In selecting a mediator to use with a particular analyte, several factors are generally relevant. For example, in the case of glucose, the mediator is selected to have a redox potential that allows it to regenerate the enzyme, glucose oxidase, from the reduced to the oxidized state. In addition, if the kinetics of the reaction with the enzyme are slow, the reduced enzyme may also react with oxygen present in this sample, leading to errors as a result in differences in hematocrit and blood pO2. (FIG. 2) Thus, it is also desirable to have fast kinetics for the enzyme-mediator reaction. It is frequently the case, however, that compounds that meet the desired criteria for redox potential and kinetics are poorly soluble in aqueous solutions, such as blood. This means that the maximum concentration of mediator is limited, and as a result that the maximum amount of signal that can be generated is limited.